RELATIONAL MODEL III

CONTENT

• Enforcing Integrity Constraints
• Querying Relational Data

Enforcing Integrity Constraints

We will consider the following constraints:

SQL NOT NULL Constraint

The following SQL enforces student_id and Firstname column to not accept NULL values:

CREATE TABLE student

               (

                Student_id int NOT NULL,

                FirstName varchar(255) NOT NULL,

                LastName,

                Age,

               Class,

               Scores

               )

SQL UNIQUE KEY Constraint

The following SQL enforces gsm_no column  not to accept duplicate values on the teachers’ table below. Meaning two teachers cannot be sharing one gsm_no

CREATE TABLE teachers

               (

                teacher_id int NOT NULL,

                FirstName varchar(255) NOT NULL,

                LastName,

                Age,

               email_address,

               gsm_no int(11) UNIQUE KEY

               )

SQL PRIMARY KEY Constraint

The following SQL constraint uniquely identifies each record in a database table. It must contain unique values. Each table should have a primary key and cannot be NULL. In most cases, the id is used as the primary key so we will enforce it on the teacher_id in the SQL syntax below.